Answer:
C. Her eyes were too earnest to be earnest.
Step-by-step explanation:
A paradox is an apparently true statement that leads to a logical contradiction, or a situation that contradicts common intuition. In simple terms, a paradox is "the opposite of what someone thinks to be the truth".
This description of what is a paradox exemplifies well what happens in the phrase "Her eyes were too earnest to be earnest."